Biosynthesis and characterization of poly(d-lactate-co-glycolate-co-4-hydroxybutyrate)

Poly(d-lactate-co-glycolate-co-4-hydroxybutyrate) [poly(d-LA-co-GA-co-4HB)] and poly(d-lactate-co-glycolate-co-4-hydroxybutyrate-co-d-2-hydroxybutyrate) [poly(d-LA-co-GA-co-4HB-co-d-2HB)] are of interest for their potential applications as new biomedical polymers. Here we report their enhanced production by metabolically engineered Escherichia coli. To examine the polymer properties, poly(d-LA-co-GA-co-4HB) polymers having various monomer compositions (3.4-41.0mol% of 4HB) were produced by culturing the engineered E. coli strain expressing xylBC from Caulobacter crescentus, evolved phaC1 from Pseudomonas sp. MBEL 6-19 (phaC1437), and evolved pct from Clostridium propionicum (pct540) in a medium supplemented with sodium 4HB at various concentrations. To produce these polymers without 4HB feeding, the 4HB biosynthetic pathway was additionally constructed by expressing Clostridium kluyveri sucD and 4hbD. The engineered E. coli expressing xylBC, phaC1437, pct540, sucD, and 4hbD successfully produced poly(d-LA-co-GA-co-4HB-co-d-2HB) and poly(d-LA-co-GA-co-4HB) from glucose and xylose. Through modulating the expression levels of the heterologous genes and performing fed-batch cultures, the polymer content and titer could be increased to 65.76wt% and 6.19g/L, respectively, while the monomer fractions in the polymers could be altered as desired. The polymers produced, in particular, the 4HB-rich polymers showed viscous and sticky properties suggesting that they might be used as medical adhesives.